Volvo Repair: air conditioning issue, v70 t5, lot air

Question
I drive a 1998 V70 T5 with 224K miles. I love this car and try to take very good care of it. This summer, the air conditioning is working well sporadically, especially if the temperature exceeds 75 degrees. Sometimes it blows warm air, especially if it is set on automatic (pushed to the left) with the temp set to blue (all the way down).  I can drive for miles with the air remaining warm and then if I turn OFF the air and wait for five minutes, it MAY become cold when I turn it back on. This is working less often than it used to so I assume something must be amiss.

I took the car to a local mechanic (not a VOlvo certified mechanic) and he did somoe pressure readings and temp readings, concluding that everything appears to be working well. As soon as I drove away, the air was blowing warm once again.    What may be the most obvious cause for the lack of cold air?

Thanks in advance for any advice you can give me.  I am so grateful for this resource!  

Answer
if you not low on  ac freon than you have to check sensor  but first chekc outside temperature sensor it is snaped in the bumper in the front of radiator if is close to radiaor ac will not work when the sensor get to hot  also i thing i you have cloged air flow thru radiator,ac condesonr,and intecooler all those parts are bolted together now when you drive all the dirt insects goes in fins of radiator.interclooler and cloges them up making air flow hard and if you dont have lot air flow thru those parts ac will not work at times do that first